Switched capacitor DC-DC converters can be a useful and advantageous alternative to inductor-based DC-DC converters. Advantages of switched capacitor DC-DC converters can include far lower electro-magnetic induction (EMI) noise, lower component height, smaller device volume, higher operating efficiency and higher power density. A shortcoming of switched capacitor DC-DC converters is that flying capacitors and load capacitors therein can draw, during power up or fast input voltage transient, high in-rush currents through one or more switched charging transistors. Such currents can exhibit magnitudes that can shorten the average life of the switched charging transistors or damage the transistors.
One technique to reduce in-rush current in switching circuits includes inserting one or more dedicated current sources within the switched capacitor DC-DC converter to provide, upon start up, a direct feed of pre-charging currents to flying and output capacitors before the main switching transistors are turned on. One disadvantage of this technique may include a long duration pre-charge time due to large output capacitors, practical limits on the size of the current source switching transistors, or both. Also, the switched DC-DC converter may not be able to deliver a sufficient load current during pre-charging of the capacitors.
Another technique used to reduce in-rush current in switching circuits includes adding a “hot-swap” input converter to convert step increases in input voltage (e.g., a step produced at power-up) to a slow ramp-up voltage. This technique requires an additional input series power transistor with high safe-operating-area and of a large size. As a result, a hot swap input converter can add significant conduction loss, and can occupy significant board real estate with high solution cost.
Accordingly, what is needed is a mechanism for effectively limiting in-rush current across switching transistors during power up or fast input voltage transients, without adding considerable power loss, solution size, cost and complexity.